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Objective: Microbiological testing evaluations of hygienic procedure properties using PALL BB22-15MS filters to prevent contamination of "single use" breathing circuits during anesthesia., Design: Prospective study., Setting: Two operating rooms in a University hospital., Patients: One hundred and thirty eight patients underwent general anesthesia for urologic surgery procedures. Patients with positive tests for HIV, B and C hepatitis and those considered to be at risk for HIV infection were excluded. The study was divided into five phases on the basis of times of usage of the same circuit for an increasing number of patients: in phase I, microbiological tests were performed on 4 circuits used on 4 different patients; in phase II the same tests were performed on 2 circuits each used on two different groups of three patients; in phase III, a circuit was used on a group of 15 patients and another on a group of 16 patients and the results were analysed; in phase IV a circuit used in a group of 32 patients was evaluated and phase V involved the analysis of a circuit that was used in a group of 65 patients., Interventions: A filter was left in place between the patient and the circuit's Y-piece during all phases of anesthesia. The level of microbial contamination of breathing circuits was analysed in order to evaluate the reliability of the procedure., Results: All analysed circuits remained uncontaminated. Staphylococcus hominis was revealed in respiratory circuit no. 6 of phase II, probably as a consequence of secondary contamination., Conclusions: Our results confirm that this procedure with the routine placement of a PALL BB22-15MS filter at the circuit's Y-piece can provide an adequate level of protection against cross-infections during anesthesia. The hygienic protocol proposed may allow the change of the anesthetic breathing only once a month.